Method and apparatus for inspection and metrology

ABSTRACT

A method of position control of an optical component relative to a surface is disclosed. The method may include: obtaining a first signal by a first position measurement process; controlling relative movement between the optical component and the surface for a first range of motion using the first signal; obtaining a second signal by a second position measurement process different than the first position measurement process; and controlling relative movement between the optical component and the surface for a second range of motion using the second signal, the second range of motion being nearer the surface than the first range of motion.

This application claims priority to European Patent Application No.15158677.3, filed on Mar. 11, 2015, which is incorporated herein in itsentirety by reference.

FIELD

The present description relates to a method and apparatus to control adistance between two objects.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In order to monitor the lithographic process, the patterned substrate isinspected and one or more parameters of the patterned substrate aremeasured. The one or more parameters may include, for example, theoverlay error between successive layers formed in or on the patternedsubstrate and/or critical linewidth of developed photosensitive resist.This measurement may be performed on a target of the product substrateitself and/or on a dedicated metrology target provided on the substrate.There are various techniques for making measurements of the microscopicstructures formed in lithographic processes, including the use of ascanning electron microscope and/or various specialized tools.

A fast and non-invasive form of specialized inspection tool is ascatterometer in which a beam of radiation is directed onto a target onthe surface of the substrate and properties of the scattered orreflected beam are measured. By comparing one or more properties of thebeam before and after it has been reflected or scattered by thesubstrate, one or more properties of the substrate can be determined.Two main types of scatterometer are known. A spectroscopic scatterometerdirects a broadband radiation beam onto the substrate and measures thespectrum (intensity as a function of wavelength) of the radiationscattered into a particular narrow angular range. An angularly resolvedscatterometer uses a relatively narrowband radiation beam and measuresthe intensity of the scattered radiation as a function of angle.

A particular application of scatterometry is in the measurement offeature asymmetry within a periodic target. This can be used as ameasure of overlay error, for example, but other applications are alsoknown. In an angle resolved scatterometer, asymmetry can be measured bycomparing opposite parts of the diffraction spectrum (for example,comparing the −1st and +1^(st) orders in the diffraction spectrum of aperiodic grating). This can be done simply in angle-resolvedscatterometry, as is described for example in U.S. patent applicationpublication US2006-066855.

SUMMARY

With reduction of the physical dimensions in lithographic processing,there is demand to, for example, increase measurement accuracy and/orreduce the space occupied by targets dedicated to metrology orinspection. Image based scatterometry measurements have been devised toallow the use of smaller targets, by taking separate images of thetarget using −1^(st) and +1^(st) order radiation in turn. Examples ofthis image based technique are described in published U.S. patentapplication publication nos. US2011-0027704, US2011-0043791 andUS2012-0044470, which are incorporated herein in their entirety byreference

Demand for further reduction in target size and for improved accuracycontinues, however, and existing techniques suffer from variousconstraints that make it difficult to maintain accuracy and/or reducethe size of the targets. Another way to improve on inspection andmeasurement techniques is to use a solid immersion lens (SIL) as theoptical element nearest the substrate surface. The extreme proximity ofthe SIL with the substrate surface (e.g., target surface) results innear-field radiation with a very high effective numerical aperture (NA)larger than 1. Using a coherent or incoherent radiation source with thisSIL allows a very small target to be inspected.

To take advantage of the increasing numerical aperture, the gap betweenthe SIL and the substrate needs to be set to a desired value. Forexample, the gap may be within the range of λ/40 to λ/8 (where λ is thewavelength of the measurement radiation) e.g., within the range of10-100 nm or 10-50 nm, to have the SIL in effective optical contact withthe substrate. An example optical gap measuring method and apparatus caninvolve detecting cross components of polarization in the high numericalaperture element. The cross polarized signal is then recorded by adetector and can be used as an input parameter into a gap controlprocess. This cross polarized signal may also be normalized by the crosspolarized signal detected at a large gap of several wavelengths. Inanother example, the gap may be controlled by reference to reflectedlaser radiation intensity. With any detecting method, the gap betweenthe SIL (or other component) and the substrate (or other surface) needsto be established to be, and maintained at, a desired gap distance ordistance range.

With such small gap distances and various surface topographies possible(whether expected or unexpected due to process variations), it isdesired to provide one or more methods and apparatus to control theposition of a component relative to a surface at solid immersion gapdistances. So, as a particular application, an embodiment may be appliedto controlling a gap between an optical element and a reflective ordiffractive surface for, e.g., inspection of a layer manufactured by alithographic technique to measure overlay error or other one or moreother parameters.

In an aspect, there is provided a method of position control of anoptical component relative to a surface, the method comprising:obtaining a first signal by a first position measurement process;controlling relative movement between the optical component and thesurface for a first range of motion using the first signal; obtaining asecond signal by a second position measurement process different thanthe first position measurement process; and controlling relativemovement between the optical component and the surface for a secondrange of motion using the second signal, the second range of motionbeing nearer the surface than the first range of motion.

In an aspect, there is provided a method of position control of anoptical component relative to a surface, the method comprising:providing radiation through the optical component to reach the surface;blocking at least part of the radiation redirected by the surface tocause a change of shape or size of illuminated area in a pupil, orconjugate thereof, as a function of change in position between theoptical component and the surface; and detecting the redirectedradiation of the illuminated area to produce a trigger signal based onwhich the position of the optical component with respect to the surfaceis controlled.

In an aspect, there is provided a method comprising: providing radiationthrough an optical component to reach a surface; causing a change ofshape or size of an area illuminated by radiation redirected by thesurface as a function of change in position between the opticalcomponent and the surface; detecting, using a detector, at least part ofthe redirected radiation after having passed through an aperture of amask to produce a detection signal, the aperture spaced apart from theintersection of an optical axis of the redirected radiation with themask; and deriving a trigger signal as a function of the detectionsignal and a filtered version of the detection signal.

In an aspect, there is provided a method comprising: providing radiationthrough an optical component to reach a surface; causing a change ofshape or size of an area illuminated by radiation redirected by thesurface as a function of change in position between the opticalcomponent and the surface; detecting, using a first detector, at leastpart of the redirected radiation to produce a first detection signal;detecting, using a second detector, at least part of the redirectedradiation to produce a second detection signal, wherein the firstdetector has a first detector radiation receiving element extending in aplane and the second detector has a second detector radiation receivingelement extending in substantially the same plane as the first detectorradiation receiving element and the first detector radiation receivingelement being generally concentric to the second detector radiationreceiving element; and deriving a trigger signal as a function of thefirst and second detection signals.

In an aspect, there is provided a method of manufacturing deviceswherein a device pattern is applied to a series of substrates using alithographic process, the method including inspecting at least a targetformed as part of or beside the device pattern on at least one of thesubstrates using a method as described herein, and controlling thelithographic process for later substrates in accordance with the resultof the method.

In an aspect, there is provided a non-transitory computer programproduct comprising machine-readable instructions for causing a processorto cause performance of a method as described herein.

In an aspect, there is provided a system comprising: an inspectionapparatus configured to provide a beam on a measurement target on asubstrate and to detect radiation redirected by the target to determinea parameter of a lithographic process; and a non-transitory computerprogram product as described herein.

In an aspect, there is provided a detection apparatus comprising: afirst mask configured to receive at least part of radiation redirectedfrom a surface and passing through an optical component moving relativeto the surface, the first mask having an aperture to allow radiation topass therethrough; a first detector configured to receive redirectedradiation passing through the first mask to produce a first detectionsignal; a second mask configured to receive at least part of theredirected radiation, the second mask having an aperture to allowradiation to pass therethrough, wherein the first mask comprises anaperture located at the intersection of an optical axis of theredirected radiation with the first mask and the second mask comprisesan aperture spaced apart from the intersection of the optical axis withthe second mask and having an inner periphery further from the opticalaxis than an outer periphery of the aperture of the first mask; and asecond detector configured to receive redirected radiation passingthrough the second mask to produce a second detection signal.

In an aspect, there is provided a detection apparatus comprising: afirst detector configured to detect radiation, the first detector havinga first detector radiation receiving element extending in a plane; and asecond detector configured to detect radiation, the second detectorhaving a second detector radiation receiving element extending insubstantially the same plane as the first detector radiation receivingelement and the first detector radiation receiving element beinggenerally concentric to the second detector radiation receiving element.

In an aspect, there is provided a detection apparatus comprising: a maskconfigured to receive at least part of radiation redirected from asurface and passing through an optical component moving relative to thesurface, the mask having an aperture spaced apart from the intersectionof an optical axis of the redirected radiation with the mask; a detectorconfigured detect at least part of the redirected radiation after havingpassed through the aperture of the mask to produce a detection signal;and a control system configured to produce a trigger signal based onwhich the position of the optical component with respect to the surfaceis controlled, the trigger signal being a function of a filtered versionof the detection signal.

In an aspect, there is provided a detection apparatus comprising: adetector configured detect at least part of radiation redirected from asurface and passing through an optical component moving relative to thesurface to produce a detection signal, wherein a shape or size ofilluminated area in a pupil, or conjugate thereof, changes as a functionof change in position between the optical component and the surface; anda processor system configured to apply a software mask having anaperture spaced apart from the intersection of an optical axis of theredirected radiation with the detector to effectively block processingof radiation received by the detector nearer to the optical axis thanthe aperture, to produce a trigger signal based on which the position ofthe optical component with respect to the surface is controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 schematically depicts an embodiment of a lithographic apparatus;

FIG. 2 schematically depicts an embodiment of a lithographic cell orcluster;

FIG. 3 schematically depicts an example inspection apparatus andmetrology technique;

FIG. 4 schematically depicts an example inspection apparatus;

FIG. 5 illustrates the relationship between an illumination spot of aninspection apparatus and a metrology/inspection target;

FIG. 6 depicts an example inspection apparatus comprising a solidimmersion lens (SIL);

FIG. 7 depicts a schematic diagram of specific components of aninspection apparatus in relation to a target surface;

FIG. 8 depicts a schematic representation of various setpoints forrelative positioning of various specific components of an inspectionapparatus in relation to a target surface;

FIGS. 9(A)-9(D) depict schematic representations of particular positionsof an objective and a solid immersion lens in relation to a radiationbeam;

FIG. 10 depicts a schematic representation of a detector systemaccording to an embodiment;

FIG. 11 depicts a simulated schematic graph of a trigger signal versusgap distance according to an embodiment;

FIG. 12(A) depicts a simulated schematic graph of a detected signal anda filtered version (low-pass in this example) of the detected signal;

FIG. 12(B) depicts a simulated schematic graph of a trigger signalderived from the detected signal and the filtered version thereof ofFIG. 11(A) according to an embodiment; and

FIG. 13 depicts a schematic representation of a detector systemaccording to an embodiment;

FIG. 14 is an enlarged detail of parts of the apparatus of FIG. 6showing an embodiment of a gap detection system;

FIG. 15 illustrates schematically a gap detection and controlarrangement in the apparatus of FIG. 14;

FIG. 16 is an enlarged detail of parts of the apparatus of FIG. 6showing a further embodiment of a gap detection system; and

FIG. 17 is a schematic flow chart of an embodiment of a method.

DETAILED DESCRIPTION

Before describing embodiments in detail, it is instructive to present anexample environment in which embodiments may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatuscomprises:

an illumination system (illuminator) IL configured to condition aradiation beam B (e.g. UV radiation or DUV radiation).

a support structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters;

a substrate table (e.g. a wafer table) WT constructed to hold asubstrate (e.g. a resist-coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and

a projection system (e.g. a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W, the projection system supported on areference frame (RF).

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, deformablemirrors, and programmable LCD panels. Masks are well known inlithography, and include mask types such as binary, alternatingphase-shift, and attenuated phase-shift, as well as various hybrid masktypes. An example of a programmable mirror array employs a matrixarrangement of small mirrors, each of which can be individually tiltedso as to reflect an incoming radiation beam in different directions. Thetilted mirrors impart a pattern in a radiation beam, which is reflectedby the mirror matrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore tables (e.g., two or more substrate tables WTa, WTb, two or morepatterning device tables, a substrate table WTa and a table WTb belowthe projection system without a substrate that is dedicated to, forexample, facilitating measurement, and/or cleaning, etc.). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure. For example, alignmentmeasurements using an alignment sensor AS and/or level (height, tilt,etc.) measurements using a level sensor LS may be made.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the patterning device and the projection system. Immersiontechniques are known in the art for increasing the numerical aperture ofprojection systems. The term “liquid immersion” as used herein does notmean that a structure, such as a substrate, must be submerged in liquid,but rather only means that liquid is located between the projectionsystem and the substrate during exposure.

Further, the lithographic apparatus may also be of a type wherein atleast an optical element is located in close proximity to a portion ofthe substrate resulting in near-field radiation spanning a gap betweenthe optical element and the substrate. This may be referred to as solidimmersion using a solid immersion lens/optical element.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder, 2-D encoder or capacitivesensor), the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the radiation beamB. Similarly, the first positioner PM and another position sensor (whichis not explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatuses to perform pre- and post-exposureprocesses on a substrate.

Conventionally these include one or more spin coaters SC to deposit oneor more resist layers, one or more developers DE to develop exposedresist, one or more chill plates CH and/or one or more bake plates BK. Asubstrate handler, or robot, RO picks up one or more substrates frominput/output port I/O1, I/O2, moves them between the different processapparatuses and delivers them to the loading bay LB of the lithographicapparatus. These apparatuses, which are often collectively referred toas the track, are under the control of a track control unit TCU which isitself controlled by the supervisory control system SCS, which alsocontrols the lithographic apparatus via lithography control unit LACU.Thus, the different apparatuses can be operated to maximize throughputand processing efficiency.

In order that a substrate that is exposed by the lithographic apparatusis exposed correctly and consistently, it is desirable to inspect anexposed substrate to measure one or more properties such as overlayerror between subsequent layers, line thickness, critical dimension(CD), etc. Accordingly a manufacturing facility in which lithocell LC islocated also typically includes a metrology/inspection system MET whichreceives some or all of the substrates W that have been processed in thelithocell. The metrology/inspection system MET may be part of thelithocell LC, for example it may be part of the lithographic apparatusLA.

Metrology/inspection results may be provided directly or indirectly tothe supervisory control system SCS. If an error is detected, anadjustment may be made to exposure of a subsequent substrate (especiallyif the inspection can be done soon and fast enough that one or moreother substrates of the batch are still to be exposed) and/or tosubsequent exposure of the exposed substrate. Also, an already exposedsubstrate may be stripped and reworked to improve yield, or discarded,thereby avoiding performing further processing on a substrate known tobe faulty. In a case where only some target portions of a substrate arefaulty, further exposures may be performed only on those target portionswhich are good.

Within a metrology/inspection system MET, an inspection apparatus isused to determine one or more properties of the substrate, and inparticular, how one or more properties of different substrates vary ordifferent layers of the same substrate vary from layer to layer. Theinspection apparatus may be integrated into the lithographic apparatusLA or the lithocell LC or may be a stand-alone device. To enable rapidmeasurement, it is desirable that the inspection apparatus measure oneor more properties in the exposed resist layer immediately after theexposure. However, the latent image in the resist has a lowcontrast—there is only a very small difference in refractive indexbetween the parts of the resist which have been exposed to radiation andthose which have not—and not all inspection apparatus have sufficientsensitivity to make useful measurements of the latent image. Thereforemeasurements may be taken after the post-exposure bake step (PEB) whichis customarily the first step carried out on an exposed substrate andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to assemi-latent. It is also possible to make measurements of the developedresist image—at which point either the exposed or unexposed parts of theresist have been removed—or after a pattern transfer step such asetching. The latter possibility limits the possibilities for rework of afaulty substrate but may still provide useful information.

FIG. 3 depicts an example inspection apparatus (e.g., a scatterometer).It comprises a broadband (white light) radiation projector 2 whichprojects radiation onto a substrate W. The reflected radiation is passedto a spectrometer detector 4, which measures a spectrum 10 (intensity asa function of wavelength) of the specular reflected radiation, as shown,e.g., in the graph in the lower left. From this data, the structure orprofile giving rise to the detected spectrum may be reconstructed byprocessor PU, e.g. by Rigorous Coupled Wave Analysis and non-linearregression or by comparison with a library of simulated spectra as shownat the bottom right of FIG. 3. In general, for the reconstruction thegeneral form of the structure is known and some parameters are assumedfrom knowledge of the process by which the structure was made, leavingonly a few parameters of the structure to be determined from themeasured data. Such an inspection apparatus may be configured as anormal-incidence inspection apparatus or an oblique-incidence inspectionapparatus.

Another inspection apparatus that may be used is shown in FIG. 4. Inthis device, the radiation emitted by radiation source 2, which may becoherent or incoherent, is collimated using lens system 12 andtransmitted through interference filter 13 and polarizer 17, reflectedby partially reflecting surface 16 and is focused into a spot S onsubstrate W via an objective lens 15, which has a high numericalaperture (NA), desirably at least 0.9 or at least 0.95. A solidimmersion inspection apparatus (using near-field radiation between anobjective of the apparatus and the target) and/or a liquid immersioninspection apparatus (using a relatively high refractive index fluidsuch as water) may even have a numerical aperture over 1.

As in the lithographic apparatus LA, one or more substrate tables may beprovided to hold the substrate W during measurement operations. Thesubstrate tables may be similar or identical in form to the substratetables WTa, WTb of FIG. 1. In an example where the inspection apparatusis integrated with the lithographic apparatus, they may even be the samesubstrate table. Coarse and fine positioners may be provided to a secondpositioner PW configured to accurately position the substrate inrelation to a measurement optical system. Various sensors and actuatorsare provided for example to acquire the position of a target ofinterest, and to bring it into position under the objective lens 15.Typically many measurements will be made on targets at differentlocations across the substrate W. The substrate support can be moved inX and Y directions to acquire different targets, and in the Z directionto obtain a desired location of the target relative to the focus of theoptical system. It is convenient to think and describe operations as ifthe objective lens is being brought to different locations relative tothe substrate, when, for example, in practice the optical system mayremain substantially stationary (typically in the X and Y directions,but perhaps also in the Z direction) and only the substrate moves.Provided the relative position of the substrate and the optical systemis correct, it does not matter in principle which one of those is movingin the real world, or if both are moving, or a combination of a part ofthe optical system is moving (e.g., in the Z and/or tilt direction) withthe remainder of the optical system being stationary and the substrateis moving (e.g., in the X and Y directions, but also optionally in the Zand/or tilt direction).

The radiation redirected by the substrate W then passes throughpartially reflecting surface 16 into a detector 18 in order to have thespectrum detected. The detector may be located in a back-projected pupilplane 11, which is at the focal length of the lens system 15, howeverthe pupil plane may instead be re-imaged with auxiliary optics (notshown) onto the detector. The pupil plane is the plane in which theradial position of radiation defines the angle of incidence and theangular position defines azimuth angle of the radiation. The detectormay be a two-dimensional detector so that a two-dimensional angularscatter spectrum of a substrate target 30 can be measured. The detector18 may be, for example, an array of CCD or CMOS sensors, and may use anintegration time of, for example, 40 milliseconds per frame.

A reference beam may be used, for example, to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton the partially reflecting surface 16 part of it is transmitted throughthe partially reflecting surface 16 as a reference beam towards areference mirror 14. The reference beam is then projected onto adifferent part of the same detector 18 or alternatively on to adifferent detector (not shown).

One or more interference filters 13 are available to select a wavelengthof interest in the range of, say, 405-790 nm or even lower, such as200-300 nm. The interference filter may be tunable rather thancomprising a set of different filters. A grating could be used insteadof an interference filter. An aperture stop or spatial light modulator(not shown) may be provided in the illumination path to control therange of angle of incidence of radiation on the target.

The detector 18 may measure the intensity of redirected radiation at asingle wavelength (or narrow wavelength range), the intensity separatelyat multiple wavelengths or integrated over a wavelength range.Furthermore, the detector may separately measure the intensity oftransverse magnetic- and transverse electric-polarized radiation and/orthe phase difference between the transverse magnetic- and transverseelectric-polarized radiation.

The target 30 on substrate W may be a 1-D grating, which is printed suchthat after development, the bars are formed of solid resist lines. Thetarget 30 may be a 2-D grating, which is printed such that afterdevelopment, the grating is formed of solid resist pillars or vias inthe resist. The bars, pillars or vias may be etched into the substrate.The pattern (e.g., of bars, pillars or vias) is sensitive to chromaticaberration in the lithographic projection apparatus, particularly theprojection system PS, and illumination symmetry and the presence of suchaberration will manifest in a variation in the printed grating.Accordingly, the measured data of the printed grating is used toreconstruct the grating. One or more parameters of the 1-D grating, suchas line width and/or shape, or one or more parameters of the 2-Dgrating, such as pillar or via width or length or shape, may be input tothe reconstruction process, performed by processor PU, from knowledge ofthe printing step and/or other inspection processes.

In addition to measurement of a parameter by reconstruction, angleresolved scatterometry is useful in the measurement of asymmetry offeatures in product and/or resist patterns. A particular application ofasymmetry measurement is for the measurement of overlay, where thetarget 30 comprises one set of periodic features superimposed onanother. The concepts of asymmetry measurement using the instrument ofFIG. 3 or FIG. 4 are described, for example, in U.S. patent applicationpublication US2006-066855, which is incorporated herein in its entirety.Simply stated, while the positions of the diffraction orders in thediffraction spectrum of the target are determined only by theperiodicity of the target, asymmetry in the diffraction spectrum isindicative of asymmetry in the individual features which make up thetarget. In the instrument of FIG. 4, where detector 18 may be an imagesensor, such asymmetry in the diffraction orders appears directly asasymmetry in the pupil image recorded by detector 18. This asymmetry canbe measured by digital image processing in unit PU, and calibratedagainst known values of overlay.

FIG. 5 illustrates a plan view of a typical target 30, and the extent ofillumination spot S in the apparatus of FIG. 4. To obtain a diffractionspectrum that is free of interference from surrounding structures, thetarget 30, in an embodiment, is a periodic structure (e.g., grating)larger than the width (e.g., diameter) of the illumination spot S. Thewidth of spot S may be over 10 or 20 μm and the target width a andlength may be 30 or 40 μm square. The target in other words is‘underfilled’ by the illumination, and the diffraction signal is freefrom interference by product features and the like outside the targetitself. The illumination arrangement 2, 12, 13, 17 may be configured toprovide illumination of a uniform intensity across a pupil plane ofobjective 15. Alternatively, by, e.g., including an aperture in theillumination path, illumination may be restricted to on axis or off axisdirections.

But, there is demand to reduce the space occupied by metrology targets.

For example, there is a desire to, for example, reduce the width of‘scribe lanes’ between target portions C on the substrate, wheremetrology targets have conventionally been located. Additionally oralternatively, there is a desire, for example, to include metrologytargets within the device patterns themselves, to allow more accuratemonitoring and correction of variations in parameters such as CD and/oroverlay. To this end, alternative methods of diffraction based metrologyhave been devised more recently. For example, in image-based metrology,two images of the target are made, each using different selected ordersof the diffraction spectrum. Comparing the two images, one can obtainasymmetry information. By selecting parts of the images, one canseparate the target signal from its surroundings. The targets can bemade smaller, and need not be square, so that several can be includedwithin the same illumination spot. Examples of this technique aredescribed in U.S. patent application publications US2011-0027704,US2011-0043791, and US2012-0044470.

In addition to or alternatively to reducing the space occupied bymetrology targets, there is demand to improve the nature of themeasurements themselves, such as their accuracy. For example, there is adesire to, for example, obtain higher sensitivity of measurement.Additionally or alternatively, there is a desire to, for example, obtainbetter decoupling between various parameters in the reconstructiondescribed above. For example, it is desired to obtain better values foreach of the specific parameters of interest, by reducing or eliminatingthe effect of measurements associated with one parameter of interestinfluencing another parameter of interest.

As the demand for size reduction and/or accuracy continues, existingtechniques may meet some technical limitations. For example, somemethods desire to capture at least the ±1^(st) diffraction orders.Taking into account the numerical aperture of the objective 15, thisconstrains the pitch (L) of a periodic structure of the target. Toimprove sensitivity and/or to reduce target size, one can consider usingshorter wavelengths A. Further, the target cannot be too small otherwiseit will not have enough features to be considered as a periodicstructure (e.g., at least 15 lines may be required which taking intoaccount previous constraints may fix the minimum periodic structure sizearound 5 μm×5 μm). Consequently, overlay, as an example, is measuredusing periodic structures features (e.g., lines) having dimensions farbigger than those of the product (e.g., device) layout, making overlaymeasurement less reliable. Ideally the feature line and pitch shouldhave similar dimensions to the product features.

FIG. 6 shows an inspection apparatus in which improvement of the natureof the measurements themselves (e.g., accuracy) and/or reduction oftarget size may be realized. In FIG. 6, a spot S′ (which may be smallerthan convention if, for example, a smaller target is desired) can beapplied to a target 30′ (which may be smaller than convention, e.g.,features of smaller pitch, if, for example, a smaller target isdesired). Like reference numerals refer to like components throughoutthe figures.

Comparing the apparatus of FIG. 6 with that of FIG. 4, a firstdifference is the provision of an additional lens element 60 close tothe target 30′. This additional lens is a miniature solid immersion lens(SIL), with a width (e.g., diameter) only on the order of a millimeter,for example in the range of 1 mm to 5 mm, for example about 2 mm. TheSIL comprises, in an example, a hemisphere of material that receivesrays of radiation at substantially normal incidence to its surface. Inan embodiment, the SIL may be a different shape such as asuper-hemisphere. In an embodiment, the SIL is made up of a material ofrefractive index n, such as glass, fused quartz, a combination ofmaterials, etc. Within the SIL material, the numerical aperture (NA) ofthe original rays is multiplied by n. The received rays come to focus atabout the center of the hemisphere or super-hemisphere and form a spotthat is smaller by a factor of n compared to what would have been in theabsence of the SIL. For example, a typical glass hemisphere having n=2will reduce the width of the focused spot by a factor of 2.

Immersion of optical elements in liquid has been used to increaseresolution in microscopy and photolithography. The solid immersion lensmay achieve similar gains without the inconvenience/problems of liquidimmersion. However, to ensure that the smaller spot size does indeedincrease the resolution of the system, the bottom of the SIL must eitherbe in contact with the target 30 or positioned extremely closely to it.This restricts its practical applications.

A so-called micro-SIL may also be used. The width (e.g., diameter) ofsuch a SIL is many times smaller, for example about 2 microns in widthinstead of about 2 millimeters. In an example where SIL 60 in the FIG. 6apparatus is a micro-SIL, it may have a width (e.g., diameter) less thanor equal to 10 μm, potentially less than or equal to 5 μm.

Whether a miniature SIL 60 or a micro-SIL lens is used, it can beattached to a movable support so that controlling the alignment andproximity to the substrate is much simpler than in the case of a lenswith bigger width. For example, the SIL 60 in FIG. 6 is mounted to aframe 62. In an embodiment, frame 62 is movable. An actuator may beprovided to move frame 62. In an embodiment, the frame 62 supports theobjective 15. Accordingly, in an embodiment, the frame 62 may move boththe objective 15 and the SIL 60 together. In an embodiment, the actuatorfor the frame 62 may be configured to move the frame 62 (and the SIL 60)in substantially the Z direction. In an embodiment, the actuator for theframe 62 may be configured to move the frame 62 (and the SIL 60) aroundthe X axis and/or Y axis. In an embodiment, the SIL 60 is in relativefixed position relative to the frame 62. This may be referred to asingle stage arrangement, where the objective 15 and SIL 60 are fixedrelative to each and are moved by the actuator of frame 62. In such acase, a benefit may be that the SIL can be mechanically positioned inthe focus of the objective.

As noted above, the SIL 60 in FIG. 6 is mounted to a frame 62, which inan embodiment supports objective 15. Of course, the SIL 60 may bemounted on a separate frame from that supporting objective 15. In anembodiment, the SIL 60 is connected to a frame (e.g., frame 62) via anarm 64 and actuator 66. Actuator 66 may be, for example, piezoelectricin operation or voice coil actuated. The arrangement where the SIL 60has an actuator to cause relative movement between a movable objective15 and the SIL 60 may be referred to as a dual stage arrangement. In adual stage, certain functionalities may be separated, e.g. separation ofmotion ranges, vibration suppression capabilities, SIL positioning andfocusing with respect to the surface. In an embodiment, the objectivestage may move only substantially in the Z-direction(substantially/essentially normal to the surface). In an embodiment, theSIL stage may move in more than 1 degree of freedom, e.g., at least 3degrees of freedom, e.g., in the Z-direction and around the X-axisand/or the Y-axis, to position the SIL substantially/essentiallyparallel to the surface. The SIL stage may not have a mechanical rangesufficient to cover the desired full travel range. So, the SIL stage canbe used to position the SIL at a certain small distance above thesurface, while the objective stage can position the objective at focuswith respect to the surface, or with respect to the SIL.

Actuator 66 may operate in combination with one or more other actuatorspositioning the objective as a whole in relation to the target. Theservo control loops of these different positioners can be integratedwith one another. The components 62, 64 and 66, together with thesubstrate table and positioners (mentioned above but not shown in FIG.6), form a support apparatus for positioning the SIL and the target T inclose proximity to one another. As noted above, in principle, SIL 60could be mounted rigidly to the frame 62, and/or may be of larger width.The separate arm and actuator allows easier control of the very smallgap, as discussed in more detail below.

Inclusion of the SIL 60 opens the possibility of focusing to a muchsmaller spot S′. The SIL works by capturing the near-field radiationfrom the target, and to this end it is positioned substantially closerthan one wavelength (λ) of radiation from the target structure,generally closer than a half wavelength, for example around λ/20. Thecloser the distance, the stronger will be the coupling of near-fieldsignals into the instrument. The gap between the SIL 60 and target 30′may therefore be less than λ/4, for example between λ/40 and λ/8.Because the NA of the inspection apparatus is effectively increased, thepitch of the target periodic structure may be reduced closer to productdimensions.

In examples where a micro-SIL would be used, incoherent radiation of thetype conventionally used in, for example, a scatterometer cannot befocused to a micron-sized spot as small as the micro-SIL. Accordingly,in such an embodiment the radiation source 2 may be changed to acoherent source. Therefore a laser source 70 is coupled to illuminationoptics 12, etc. via an optical fiber 72. The limit on the spot size onthe substrate is set by the numerical aperture of the focusing lenssystem and the laser wavelength. As an additional benefit of usingspatially coherent radiation, the instrument with laser radiation source70 can be used to perform different types of scatterometry ormeasurement. For example, coherent Fourier scatterometry (CFS) may beused to measure the target.

As highlighted above, a small gap should be maintained between the SILand the target. As also highlighted above, known techniques forcontrolling the gap have limitations, particularly when a variety ofdifferent target structures and materials are to be inspected.

For example, a significant challenge is to control a relatively smallsolid immersion lens (SIL) with a gap selected from the range of betweenλ/40 and λ/4, e.g., 10-100 nm between the SIL and the measured surfacewith a small (e.g., about 1-10% of the gap size) servo error, subject topossibly much larger vibrations caused by external disturbances, e.g.,vibrations of up to 300 nm. This may be achieved with a high-bandwidthcontrol using a signal representative of the gap distance, e.g., a gaperror signal (GES).

A “dual stage” concept may be used to facilitate positioning of the SILand the objective close to the measured surface and allows for certainfunctionalities to be separated, e.g. separation of motion ranges,vibration suppression capabilities, and/or SIL positioning and focusingwith respect to the surface. Referring to FIG. 7, an embodiment of a“dual stage” concept is schematically depicted. A SIL 60 is attached toa movable support 700 to facilitate controlling the alignment andproximity of the SIL 60 to the measured surface, in this case thesubstrate W. This may be termed the SIL stage. Further, an objective 15is attached to a movable support 710 to facilitate controlling thealignment and proximity of the SIL 60 and the objective 15 to themeasured surface, in this case the substrate W. This may be theobjective stage.

An actuator 720 may be provided to move the movable support 700 and theSIL 60 with respect to the movable support 710 and/or objective 15. Anactuator 730 may be provided to move the movable support 710 andobjective 15 with respect to a support 740. In this embodiment, themovable support 700 is mounted on the movable support 710 and somovement of the movable support 710 may also cause the movable support700 and/or the SIL 60 to move. Accordingly, in an embodiment, themovable support 710 may move both the objective 15 and the SIL 60together. Actuator 720 and/or 730 may be, for example, piezoelectric inoperation or voice coil actuated.

The SIL stage may be mechanically suspended relative to the objectivestage, which is represented by an equivalent spring and/or damping 750.The spring and/or damping 750 may be incorporated in the actuator 720and/or provided separately by appropriate spring and/or damperstructure. Similarly, the objective stage may be mechanically suspendedrelative to the support 740, which is represented by an equivalentspring and/or damping 760. The spring and/or damping 760 may beincorporated in the actuator 730 and/or provided separately byappropriate spring and/or damper structure.

In an embodiment, the actuator 720 may be configured to move the movablesupport 700 (and the SIL 60) in substantially the Z direction. In anembodiment, the actuator 720 may be configured to move the movablesupport 700 (and the SIL 60) around the X axis and/or Y axis. In anembodiment, the actuator 730 may be configured to move the movablesupport 710 (and the objective 15) in substantially the Z direction. Inan embodiment, the actuator 730 may be configured to move the movablesupport 710 (and the objective 15) around the X axis and/or Y axis. Inan embodiment, the objective stage may move only substantially in theZ-direction (substantially normal to the surface). In an embodiment, theSIL stage may move in more than 1 degree of freedom, e.g., at least 3degrees of freedom, e.g., in the Z-direction and around the X-axisand/or the Y-axis, to position the SIL substantially parallel to thesurface. The SIL stage may not have a mechanical range sufficient tocover the desired full travel range. So, the SIL stage can be used toposition the SIL at a certain small distance above the surface, whilethe objective stage can position the objective at focus with respect tothe surface, or with respect to the SIL.

Further, in an embodiment, the surface W itself may be moved. Forexample, a substrate table WT having the surface W may move the surfaceW relative to the SIL 60 to facilitate establishing an appropriate gapbetween the SIL 60 and the surface W.

To enable such positioning, one or more signals may be provided. Forexample, one or more signals 770 may be provided to enable positioningof the objective 15 and/or SIL 60 relative to the support 740 and/or tothe surface W. Similarly, one or more signals 780 may be provided toenable positioning of the SIL 60 relative to the objective 15 and/or tothe surface W. One or more signals 785 may be provided to enablepositioning of the SIL 60 relative to the surface W. As an example, asignal 770 to enable relative positioning between the objective 15 andthe support 740 may be provided by an encoder, a gas sensor, or aninterferometer. As described in more detail below, a signal 770 toenable relative positioning between the objective 15/SIL 60 and thesurface W may be a signal derived from a radiation beam 790 passingthrough the objective 15, the SIL 60 and onto the surface W. Theradiation beam 790 may be a dedicated beam for determining the positionor may be the beam used to measure the surface but used for a certaintime as a position measuring beam. A signal 780 to enable relativepositioning between the objective 15 and the SIL 60 may be a focus errorsignal (FES). A signal 785 to enable relative positioning between theSIL 60 and the surface W may be a gap error signal (GES) as describedherein.

So, the actuators 720 and 730 may operate in combination to position theobjective 15 and the SIL 60 in relation to the surface W to establish adesired gap 795. A control system is provided to control positioning ofthe SIL 60 close to the surface W and to maintain the SIL 60 at oraround that position. The control system may receive a setpoint gapvalue and control one or more actuators (e.g., actuators 720 and/730) toposition, in one or more motions, the SIL 60 at or near the setpoint gapvalue and maintain the SIL 60 at or around that position. There may besignificant relative vibrations between the surface W and the SIL 60.So, the SIL 60 may be controlled via a high-bandwidth (e.g., 1-10 kHz)feedback control system. To enable the control by the control system,the gap between the SIL 60 and the surface W may be represented by oneor more signals, e.g., a gap error signal (GES). Various techniques formeasuring the GES or other position signals are known in the art.

In an embodiment, the actuator 720 may be considered a fine positionerand the actuator 730 may be considered a coarse positioner. In anembodiment for motion in the Z-direction (e.g., vertical motion), a“dual stage” system may enable control of both the (1) focus between theobjective 15 and the SIL 60, and (2) the gap 795 between the SIL 60 andthe surface W.

Further, a “dual stage” system can enable a relatively large dynamicrange for the gap 795, e.g., about mm range with sub-10 accuracy.Referring to FIG. 8, an embodiment of Z-direction motion set points isschematically described. A first setpoint distance 800 may be definedfor the distance of the SIL 60 from the surface W (i.e., gap 795) toenable exchange of a surface to be measured (e.g., substrate W) withanother surface to be measured. In an embodiment, the first setpointdistance 800 may be selected from the range of about severalmillimetres, e.g., about 1-5 mm, or about 1 mm. Once a surface W to bemeasured is in place, the SIL 60 may be positioned closer to the surfaceWin an approach motion 805 to a second setpoint distance 810 of the gap795. In an embodiment, the second setpoint distance 810 may be selectedfrom the range of about several hundreds of microns, e.g., 400 to 150microns, e.g., about 250 to 350 microns, e.g., about 300 microns. Thesecond setpoint distance 810 enables relatively safe relative movementbetween the surface W and the SIL 60, for example, to horizontallyposition the SIL 60 over a target 30.

From the second setpoint distance 810, the SIL 60 may be positionedcloser to the surface W in an approach motion 815 to a third setpointdistance 820 of the gap 795. In an embodiment, the third setpointdistance 820 may be selected from the range of half a wavelength, e.g.,about 350 to 125 nanometers, e.g., about 350 to 175 nanometers, e.g.,about 300 nanometers. The third setpoint distance 820 may be the maximumgap 795 for which the GES can be used.

From the third setpoint distance 820, the SIL 60 may be positionedcloser to the surface W in an approach motion 825 to a fourth setpointdistance 830 of the gap 795. In an embodiment, the fourth setpointdistance 830 may be selected from the range of about 100 to 10nanometers, e.g., about 50 to 10 nanometers, e.g., about 20-30nanometers or about 30 nanometers. The fourth setpoint distance 830 maybe the gap 795 at which the measurement is taken 835. During themeasurement, the gap 795 is substantially maintained at the fourthsetpoint distance 830.

Once the measurement is complete, the SIL 60 is positioned further awayfrom the surface W to either enable a further measurement at anotherlocation on the surface or exchange of the surface W for another surfaceW. In an embodiment, the SIL 60 is positioned further away from thesurface W in a retraction motion 840 to a third setpoint distance 820,which may have the same value as for the approach motion 825 or may bedifferent therefrom. From the third setpoint distance 820, the SIL 60 ispositioned further away from the surface W in a retraction motion 845 toa second setpoint distance 810, which may have the same value as for theapproach motion 815 or may be different therefrom.

As noted above, the SIL 60 may be maintained at the second setpointdistance 810 to enable relatively safe relative movement 855 between thesurface W and the SIL 60 to, e.g., horizontally position the SIL 60 overa further target 30 by relative movement between the SIL 60 and thetarget (e.g., moving the surface W horizontally and/or moving the SIL 60horizontally). So, in an embodiment, for each target at a differentlocation on the surface W, the approach motions 815 and 825 andretraction motions 840 and 845 of the SIL is repeated to help avoiddamage of the surface W and the SIL 60 during relative motion betweenthe SIL 60 and the surface W. In an embodiment, the retraction motions840 and 845 may be combined into a single motion to the second setpointdistance 810, where, for example, the next operation is relativemovement 855 between the surface W and the SIL 60 to position the SIL 60over a further target 30.

If the surface W is being replaced with another surface W or the sensoris being shut down, the SIL 60 is positioned further away from thesurface W in a motion 850 to a first setpoint distance 800, which mayhave the same value as for the start of the motion 805 or may bedifferent therefrom. In an embodiment, the motions 840, 845 and 850 maybe combined into a single motion to the first setpoint distance 800,where, for example, the next operation is the surface W being replacedwith another surface W or the sensor being shut down.

In an embodiment, the approach motion 805 need not have the sameparameters (e.g., acceleration, speed, setpoint, etc.) as the retractionmotion 850. Similarly, in an embodiment, the retraction motion 845 neednot have the same parameters (e.g., acceleration, speed, setpoint, etc.)as the approach motion 815. Similarly, in an embodiment, the retractionmotion 840 need not have the same parameters (e.g., acceleration, speed,setpoint, etc.) as the approach motion 825.

These various motions take time due to, e.g., inertia of moving partsand limitations of the actuator and/or its amplifier. To improveproductivity, it is desirable to reduce the time taken within the limitsand constraints of the sensor system, the small distances, the controlsystem bandwidth, etc. In particular, “extra” time in the motions 815,825, 840 and 845 can significantly impact productivity (e.g., number oftargets measured per minute).

In an embodiment, the approach velocity in motion 815 may be limitingfor the productivity (as well as the approach velocity in motion 805,although the motion 805 occurs less frequently than the approach motion815). For example, the GES may only be usable to the outer limit of anear-field gap distance (e.g., about 350 to 125 nanometers, e.g., about300 nm), so the available “braking” distance before the SIL would impactthe surface W is relatively short, e.g., a fraction of the about 350 to125 nanometers, e.g., about 300 nm. So, given that “brake” distance andother conditions of the system, a maximum allowable approach velocityfor motions 805 and 815 is determined, e.g., about 100-1000 μm/s, e.g.,250-350 μm/s or about 300 μm/s. So, since the GES may not be usableoutside a near-field gap distance, the relative motion between the SIL60 and surface W would be with that maximum velocity over the full rangefrom the first setpoint distance 800 at the start of each surface W andfrom the second setpoint distance 810 between targets on a surface W.So, it is desirable to enable a higher velocity at least in motion 815.

Accordingly, in an embodiment, there is provided a multi-step “braking”process. That is, in an embodiment, the relative motion between the SIL60 and the surface W is “braked” in two or more steps. In a first step,a “far-field braking” is applied using a trigger signal in the range tothe second setpoint distance 810 and/or to the third setpoint distance820. At the third setpoint distance 820, a “near-field braking” isapplied by the use of, e.g., the GES signal. With such an approach, thevelocity in motion 805 and/or motion 815 can be increased by, e.g., afactor of about 10 times to, e.g., about 1-10 mm/s, e.g., 2.5 to 5 mm/s,e.g., about 3 mm/s. The new maximum allowable velocity may be determinedby the brake distance needed due to inertia of the applicable componentsand by the power electronics (e.g., the brake distance may not exceedthe range of the SIL stage). For example, the multi-step brake processmay reduce the time for motion 815 by a factor of about 5 times.

In an embodiment, the trigger signal is an optical signal. In anembodiment, radiation 790 that propagates through the objective 15 andthe SIL 60, and that is redirected by the surface W and returns throughthe objective 15 and the SIL 60, is used as the basis for the opticaltrigger signal. So, with such a signal, the impact on the overall systemdesign is relatively small by using illumination that is alreadyavailable for, e.g., other control signals, and by using a relativelysimple detection method, which has a low impact on the optical path.

Referring to FIG. 9, schematic representations of particular positionsof an objective 15 and a SIL 60 in relation to a radiation beam 790 aredepicted for determination of a trigger signal. That is, FIGS. 9(A)-(D)show possible ray paths through the objective 15 and the SIL 60. Thepupil plane 900 and the focal plane 910 of the objective 15 are depictedas well as the critical angles (CA). In an embodiment, the SIL 60 ishemispherical as shown in FIG. 9. Further, in an embodiment, the SIL 60has a tip positioned in a central part of the bottom side of the SIL.The tip enables a small gap 795. As discussed further below, the SIL 60does not need to have a tip as shown. Further, during the relativemovement between the SIL 60 and the surface W, the SIL 60 is kept on thefocal plane of the objective 15. Also, for ease of depiction, FIG. 9only illustrates a critical ray at different gaps, in order toillustrate that when the gap reduces, the illuminated radius on thepupil plane increases. Other rays, besides the critical rays depicted,would be provided in a practical implementation of the optical system ofFIG. 9.

Depending on its position in the pupil plane, a ray of beam 790 canpropagate in different ways through the objective 15. Referring to FIG.9(A), the objective 15 is critically illuminated, and the pupiloverfilled. All rays of beam 790 propagate down from the pupil plane 900towards the SIL 60 and the surface W. If a ray is sufficiently close tothe optical axis OA, its redirection from the surface W will be on thetip of the SIL 60, and this ray will propagate back through theobjective 15. Referring to FIG. 9(B), showing a gap 795 smaller indistance than in FIG. 9(A), there is a critical position on the pupilplane 900 for which the redirected ray from the surface W illuminatesthe edge of the tip of the SIL 60 and propagates back through theobjective 15.

Referring now to FIG. 9(C), showing a size of the gap 795 smaller thanin FIG. 9(B), the ray of beam 790 is beyond a critical position, such asthe critical position in FIG. 9(B). This ray is vignetted or otherwisecompletely or partially blocked in the system. This vignetting/blockingcan have several causes. An example reason for such vignetting/blockingis due to the beam redirected by the surface W no longer reaching thetip of the SIL 60 as shown in FIG. 9(C). Alternatively or additionally,it could occur further downstream in the optical system. The point isthat as long as it is systematic, and does not occur while the surface Wis in the focal plane of the objective 15 (i.e., it doesn't occur whenthe surface W comes into optical contact with the SIL 60, which is inthe focal plane), it does not matter what exactly causes thevignetting/blocking. Further, referring to FIG. 9(D), showing a size ofthe gap 795 smaller than in FIG. 9(C), a ray can be beyond the criticalangle CA in the pupil plane 900, and hence, doesn't reach the surface Wdue to total internal reflection on a surface of the SIL 60.

Because the vignetting/blocking is a property of the optical system, andthe optical system is designed and constructed to be stable, the fillingof the pupil due to the changing in the size of the gap 795 is highlyrepeatable, and hence suitable to use for the construction of thetrigger signal. So, by reducing the size of the gap 795, the thresholdfor rays that will propagate back into the SIL 60 will move. So, forexample, during the reduction in size of the gap 795, between theoptical axis OA and the critical angle CA, the pupil plane will begradually filled with radiation in a central portion, which radiation isredirected from the surface W.

Now, referring to FIG. 10, a schematic representation of a detectorsystem according to an embodiment is depicted that takes advantage ofthe radiation behaviors in FIG. 9. The detector system comprises a beamsplitter 1000 to receive the radiation from the objective 15 (objective15 and the remainder of the optical system is not shown to enableclearly showing of the components in FIG. 10), whether redirected bysurface W, internally reflected, etc. The beam splitter 1000 provides aportion of the radiation towards detector 1005 and a portion of theradiation towards detector 1020. Either or both detector 1005 anddetector 1020 may be a photodiode, or a camera sensor. An optical system(not shown) projects the pupil plane 900 of the objective 15 onto aconjugate plane 1015, which is adjacent the detector 1005, and aconjugate plane 1030, which is adjacent the detector 1020.

At or near the conjugate plane 1015, there is provided a mask 1010.Similarly, at or near the conjugate plane 1030, there is provided a mask1025. In an embodiment, masks 1010 and 1025 are different. Exampleformats of the mask 1010 and 1025 are shown in the form of theirtop/bottom views. In an embodiment, the masks 1010, 1025 block all theincident radiation except for radiation that fits within a part lefttransmissive. In an embodiment, the transmissive parts on mask 1010 andmask 1025 have different radii or widths, with the transmissive part ofmask 1010 having a smaller radius or width than the transmissive part ofmask 1025. In an embodiment, mask 1010 may comprise a plate with atransmissive opening (e.g., a circular hole such as an open aperture) ina central portion of the plate and mask 1025 may comprise a plate with atransmissive ring opening (e.g., an annulus such as annulus openaperture) around a central portion of the plate.

Referring to the right hand side of FIG. 10, the pupils for threedifferent distances of gap 795 are schematically shown, where the lightportions correspond to the radiation reaching the pupil and the darkportions correspond to the empty pupil (i.e., absence of radiation). Thepupil 1035 corresponds roughly to the state in FIG. 9(D), where the gap795 is relatively large. In this pupil 1035, a highly illuminated ringis due to the internal reflection of the SIL 60 beyond the criticalangle CA. The pupil 1040 corresponds roughly to a state in FIG. 9(C),where the gap 795 is smaller than in FIG. 9(D). So, a significantproportion of the radiation is still at or outside the critical angle CAand so illuminates the ring. Another proportion within the criticalangle CA comes within the critical position of the vignetting/blockingdescribed above and thus is redirected through SIL 60 toward thedetectors 1005, 1020. This radiation fills in a central portion of thepupil. The pupil 1045 corresponds roughly to another state in FIG. 9(C),where less of the vignetting/blocking occurs and more of the radiation795 is incident on the surface W and returns back all the way throughthe optical system to the detectors. The pupil 1045 may be designated asa pupil for a trigger distance, i.e., the point at which a motion may beterminated or begin to be terminated. As the gap 795 is further reduced,more of the central portion of the pupil would become illuminated untilthere is no effect of the vignetting/blocking (at the transition from,e.g., FIG. 9(C) to FIG. 9(B)) and a solid illumination pupil is formed(corresponding to, e.g., FIG. 9(A)).

So, as the gap 795 is reduced, the pupil plane (angle <critical angleCA) will gradually be filled with radiation, as explained above. Nowreferring to the masks 1010 and 1025 in FIG. 10, from a certain fillingin of a central portion of the pupil onwards, the transmissive part(e.g., hole) on mask 1010 is filled with radiation (and thus detector1005 is illuminated and emits a signal), but the transmissive part(e.g., the annulus) of mask 1025 is not. Only if and when the gap 795 isclosed further, radiation eventually fills the transmissive part (e.g.,the annulus) of mask 1025 (and thus detector 1025 is illuminated andemits a signal). A ratio between the signals of detectors 1005 and 1020may be used to establish a trigger signal (TS) as follows:

$\begin{matrix}{{TS} = \frac{V_{{PD}\; 2}}{V_{{PD}\; 1} + ɛ}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

where V_(PD2) is a signal from detector 1020 (e.g., a voltage signal),V_(PD1) is a signal from detector 1005 (e.g., a voltage signal), and theterm E term has a positive value in the order of the noise level ofV_(PD1) to avoid a divide-by-zero problem since V_(PD1)≧0. Othermathematical equations based on V_(PD1) and V_(PD2) could also besuitable as a trigger signal.

The trigger signal can be monitored to determine whether a certain gapdistance 795 is obtained. Referring to FIG. 11, a simulated schematicgraph of a trigger signal versus gap distance is depicted. In thisgraph, three general regions can be seen. In a first region, when thedistance of the gap 795 begins to be reduced, the distance of the gap795 is relatively large and only radiation near to the optical axis willbe able to propagate back from the surface W to the detectors (e.g.,pupil 1035 of FIG. 10). This radiation is weak and only detected bydetector 1005 (since mask 1010 has an opening on the optical axis) andso V_(PD1) will have a relatively small value and V_(PD2) will be zerosince mask 1025 is substantially opaque on the optical axis. Thus, thetrigger signal TS will be in the region marked 1130. The trigger signalTS will be in region 1130 as long as V_(PD2) is zero, irrespective ofthe value of V_(PD1). Further, the trigger signal TS will be in region1130 as long as the value of V_(PD1) is relatively larger compared tothe value of V_(PD2).

So, if the gap 795 decreases, the value of V_(PD1) will become largerand, due to the small width (e.g., diameter) of the transmissive part ofthe mask 1010, reach its maximum over a short change in distance of gap795. But, since V_(PD1) is in the denominator of Equation (1), thetrigger signal TS remains 0 as long as V_(PD2) is zero. Once the gap 795decreases to a size at which radiation also fills the transmissive partof mask 1025, the trigger signal TS will increase in value in transitionregion 1110. It may reach its maximum quickly in region 1110. Once themaximum values of V_(PD1) and V_(PD2) are reached, i.e., thetransmissive parts of masks 1010 and 1025 are fully illuminated withstrong radiation, the trigger signal TS will reach its maximum value inregion 1120 and remain there for the remainder of the reduction in sizeof the gap 795. Thus, the region 1130 represents when the distance ofthe gap 795 is further away from a desired trigger distance of the gap795 in the transition region 1110, the region 1110 is a small transitionrange in which the gap size is at, or reaches, the desired triggerdistance of gap 795, and the region 1120 represents when the distance ofthe gap 795 is smaller than the desired trigger distance. So, as shownin FIG. 11, during the reduction in size of the gap 795, the triggersignal TS will show a step-like response making it suitable as a triggerwhen the trigger signal exceeds, equals or is lower than a certainthreshold.

As will be appreciated, a similar graph would be obtained with increasein size of the gap 795 from a small distance of gap 795 to a largerdistance. The region 1120 would correspond to the small size of gap 795and then the signal TS would decrease in transition region 1110 as thesize of the gap 795 increases until the signal TS is in region 1130 fora relative larger size of gap 795.

So, as shown in FIG. 11, a threshold 1100 may be defined to have a valuein the transition region 1110 and against which the trigger signal TSmay be evaluated. For example, for a motion of reducing the size of gap795, the trigger signal TS may be evaluated as to whether the value isgreater than or equal to threshold 1100. When it is greater than orequal to the threshold 1100, an action can be triggered, such as thestop of the relative motion or the beginning of stopping the motion.Similarly, for a motion of increasing the size of gap 795, the triggersignal TS may be evaluated as to whether the value is less than or equalto the threshold 1100. When it is less than or equal to threshold 1100,an action can be triggered, such as the stop of the relative motion, thebeginning of stopping the motion, or acceleration of the motion.

To obtain a desired trigger distance of gap 795, the threshold 1100 maybe appropriately selected to the desired distance of gap 795 in thetransition region 1110 (see FIG. 11). To obtain the actual gap distancethat corresponds to the different values of the trigger signal TS, thedistance of gap 795 may be separately measured in coordination withdetermining values of the trigger signal TS (and/or values of V_(PD1)and of V_(PD2)) in a calibration step or may be mathematicallycalculated from first principles or by simulation. Further, to shift thetransition region 1110 laterally on the graph of FIG. 11, i.e., toobtain a different range of values of the distance of gap 795 covered bytransition region 1110, the design of the transmissive parts of themasks 1010 and 1025 may be altered, e.g., in size and/or shape (also by,for example, a calibration step or by mathematical calculations fromfirst principles or by simulation). In an embodiment, additionally oralternatively, to shift the transition region 1110 laterally on thegraph of FIG. 11, i.e., to obtain a different range of values of thedistance of gap 795 covered by transition region 1110, the(de)magnification of the conjugated pupil plane may be adjusted (alsoby, for example, a calibration step or by mathematical calculations fromfirst principles or by simulation).

By using detectors 1005 and 1020, it may avoid directly comparing anamount of reflected radiation to a preset threshold, and with that, mayavoid a direct dependence on the reflectivity of the surface W. Thisreflectivity can vary an order of magnitude, depending on thestructures, materials, etc. present on the surface W. So, working withdetectors 1005 and 1020, and evaluating their signals with respect toeach other, helps make the system more robust against processvariations.

In an embodiment, a trigger signal may be generated with one maskeddetector (e.g., a photodiode) at an image of the pupil of the objective15. In particular, in an embodiment, a detector with a transmissive ringpart in an opaque region is used similar to the detector 1020 and itsmask 1025 of FIG. 10. An advantage of this measurement scheme is thatthe optical path may have only one detector and associated masking. Toprovide process variation robustness, the single measured signal (e.g.,V_(PD2)) may be compared with a filtered version of the same signal. Inan embodiment, the filtered version may be a low-pass filtered versionof the detected signal. If a sudden change in radiation amount at thedetector is detected, the detected signal of the detector will changeimmediately, while a low-pass version of the same signal lags behind.The filtered version of the same signal can be generated electronicallyor digitally. So, the trigger signal TS may be defined as:

$\begin{matrix}{{TS} = \frac{V_{PD}}{V_{filtered} + ɛ}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

where V_(PD) is the detected signal of the detector, V_(filtered) is thefiltered version of the detected signal, and the term ε term has apositive value in the order of the noise level of V_(PD) to avoid adivide-by-zero problem since V_(filtered)≧0. Other mathematicalequations based on V_(PD) and V_(filtered) could also be suitable as atrigger signal. For example, further formulations include

${{TS} = {V_{PD} - V_{filtered}}},{{TS} = \frac{V_{PD}}{V_{lowpass} + ɛ}},{or}$TS = V_(highpass).

Referring to FIG. 12, signal 1200 is the detected signal of the detectoragainst the distance of the gap 795. In the example of FIG. 12, signal1210 is a low-pass version of the detected signal against the distanceof the gap 795. As can be seen in the graph, the signal 1210 slightlylags the signal 1200. This lag is taken advantage of in Equation (2) toproduce trigger signal 1220. So, the trigger signal 1220 will be zero,unless a sudden change of radiation intensity is detected at thedetector. This sudden change can be the trigger to cause an action asdescribed above when the trigger signal equals, exceed or is lower thana certain threshold. The threshold should be carefully selected to berobust against variations in the surface W reflectivity andapproach/retract velocity, which determine the trigger signal 1220 peak,and to be robust against measurement noise. The cut-off frequency of thefilter should be carefully selected as a function of the approachvelocity and the desired width of the TS signal 1220, i.e., the gaprange for which TS is larger than zero.

Like in the embodiment of FIGS. 9-11, to obtain a desired triggerdistance of gap 795, the threshold for trigger signal 1220 may beappropriately selected to the desired distance of gap 795 in the peak(see FIG. 12). To obtain the actual gap distance that corresponds to thedifferent values of the trigger signal, the distance of gap 795 may beseparately measured in coordination with determining values of thetrigger signal (and/or values of V_(PD)) in a calibration step or may bemathematically calculated from first principles or by simulation.Further, to shift the peak of signal 1220 on the graph of FIG. 12, i.e.,to obtain a different range of values of the distance of gap 795 coveredby the peak, the design of the transmissive part of the mask may bealtered, e.g., in size and/or shape and/or the (de)magnification of theconjugated pupil plane may be adjusted (both or either also by, forexample, a calibration step or by mathematical calculations from firstprinciples or by simulation).

In an embodiment, the detectors having differently shaped masks in frontof them or the single detector having a mask in front of it may bereplaced by a fast detector (e.g., a fast camera) and the “masking” maybe performed in software by image processing. Thus, this concept mayessentially be the same as the embodiments with one or more detectorshaving one or more hardware masks, with the one or more masks beingimplemented in software in combination with a pixelated detector. So, inthis embodiment, a fast detector records the same illumination profilethat falls on a detector placed at e.g. the position of detector 1020and/or detector 1005 in FIG. 10. In this embodiment, the mask 1025and/or 1010 will not be present as physical structures in front of thedetector(s), but rather implemented in software with the same shapeconsiderations as the physical masks described above. In an embodiment,the software mask(s) has pixel values of “1” at the transmitting part ofa physical hardware mask and has pixel values of “0” at the blockingpart of the physical hardware mask. So, in an embodiment, the softwaremask will then be multiplied with the illumination distribution recordedby the detector pixel by pixel, essentially turning off the pixelscorresponding to the blocking part of the applicable mask and keeping onthe pixels at the transmitting part of the applicable mask. Thus, thesoftware mask essentially turns the desired pixels off in the recordeddetector reading so that the remaining pixels may be used as a detectionsignal (and processed as described herein).

In an embodiment, the trigger signal may be generated using twoconcentric detectors (e.g., photodiodes) at an image of the pupil of theobjective 15. Referring to FIG. 13, there is provided two concentricdetectors 1300 and 1310 (e.g., photo diodes). The two detectors 1300,1310 may be insulated from each other, as shown in FIG. 13. So, if thecenter is illuminated, the inner detector 1300 will detect theillumination and give an output signal 1320. When the outer detector1310 is illuminated, it will give a signal 1330. So, the detection ofthe gradual filling of the pupil, like described in FIG. 9, can beachieved by using the concentric detectors 1300, 1310 instead of a mask.By using a structured detector like this, the radiation path of FIG. 10can be simplified by eliminating the beam splitter 1000 and the masks1010 and 1025. The two signals that are generated by this detectorarrangement are similar to the signals measured by detectors 1005 and1020 in the embodiment of FIG. 10, and hence the trigger signal may bederived in a similar fashion as described above and evaluated in asimilar manner. Instead of changing the size and/or shape of the masks1010 and 1025 to change the range of size of the gap 795 measured, thesize and/or shape of the detectors 1300 and 1310 may be altered toachieve the same effect. Additionally or alternatively, to change therange of size of the gap 795 measured, the (de)magnification of theconjugated pupil plane may be adjusted. An advantage of the simplifiedradiation path includes, for example, a more compact design and, becausethe radiation is not split over multiple arms, the signal detectedshould be stronger, and therefore, the signal-to-noise ratio should bebetter.

FIG. 14 shows a schematic partial enlarged view of parts close to thetarget, in the apparatus of FIG. 6. FIG. 14 in particular provides aschematic view of example optical paths for use in determining andcontrolling the gap (i.e. the distance of gap 795) in the apparatus ofFIG. 6. FIG. 15 shows schematically an embodiment of a gap determiningand controlling system. With regard to the function of the apparatus asa metrology or inspection apparatus, a measurement illumination beam1400 follows an illumination path comprising optical components 12 (notshown in FIG. 14 for convenience), 13 (not shown in FIG. 14 forconvenience), 15, 16 (not shown in FIG. 14 for convenience), 17 (notshown in FIG. 14 for convenience), and 60 described above with referenceto FIG. 6, and thus will not be discussed here. A collection pathcomprising optical components 60, 15 to collect radiation redirected bytarget 30′ is also described above with reference to FIG. 6. Theradiation collected by optical components of the collection path isdirected to a detector 18 (not shown in FIG. 14 for convenience)connected to processor PU (not shown in FIG. 14 for convenience) fortarget reconstruction or other purposes. As mentioned above, an exampleapplication of these parameters may be for determining overlay error.Target 30′ may be formed on a substrate W that has been patterned andprocessed using the lithographic apparatus of FIG. 1 and the cluster ofprocessing tools described above with reference to FIG. 2. The techniquedisclosed in the present disclosure is not limited to such inspectionapparatus. In another application, for example optical recording,illumination paths and collection paths may be similarly arranged.

In an embodiment, to determine and control the gap 795, a radiation beam1405 (e.g., a broadband radiation beam) follows an optical path thatwill be referred to as the control path. Beam 1405 may be referred to asa control beam and may be beam 790 described herein, a beam to arrive atthe GES, and/or other beam used to determine a distance or position. Thecontrol path in this example comprises optical components 1410 and 1420,which may take the form of mirrors or partially-reflective surfaces.Control beam 1405 is directed to SIL 60 by optical component 1410through optical component 1420. Control beam 1405 may comprise a narrowbeam of broadband radiation that passes through SIL 60 to impinge ontarget 30′ at substantially/essentially normal incidence to thesubstrate surface. The control beam radiation redirected by target 30′is labeled 1425 and is directed by optical component 1420 to a detectionarrangement (not shown in FIG. 14 but some examples of which are shownin FIGS. 10 and 13). An aperture 1415 may be placed in the control pathto reduce the width of the control beam 1405 so as to increase thecoherence length. The width of control beam 1405 may also be varied, forinstance, by an optical coating on optical component 1420 so as toselect part of the radiation impinging on optical component 1410. Anaperture stop 1430 may also be placed in the control path to select aportion of radiation 1425 that is delivered to the detectionarrangement.

For convenience of description, the source to generate control beam 1405is not shown in FIG. 14. A radiation source emitting radiation of one ormore wavelengths selected from the ranging of 400 to 900 nm may be used.The source may be, for example, a lamp emitting white light or aso-called white light laser. In other embodiments, the radiation may bepolychromatic (comprising many individual wavelengths), rather thanhaving a continuous broad spectrum. The source of the measurementillumination beam 1400 and control beam 1405 may be one and the same. Inone such embodiment the laser source 70 of FIG. 6 may be replaced by abroadband light source to supply radiation for both beams 1400 and 1405,when the application does not require the use of a highly coherent lightsource. Alternatively, different sources may be used to generate beams1400 and 1405.

FIG. 15 illustrates schematically an arrangement to monitor and controlthe value of size of gap 795. The arrangement of FIG. 15 includes adetector arrangement 1435 (e.g., such as described above). Radiation1425 is directed to detector arrangement 1435. The one or more signalsproduced by detector arrangement 1435 are directed to processor system1440 which communicates with a processor PU. Processor system 1440analyzes the one or more signals produced by the detector arrangement todetermine, e.g., a distance value of gap 795, a brake, trigger or othersignal as described herein and/or one or more setpoints for movement ofa component. In an embodiment, such analysis may be performed byprocessor PU. Processor PU then uses the results of the determination tocontrol the value of gap 795 to a desired set point by activating one ormore actuators (e.g., actuator 66). In this way, servo control of thegap 795 is achieved.

Further, SIL 60 may be irradiated obliquely by control beam 1405 at anincident angle different from zero. The optical arrangement of FIG. 14to illuminate the target and collecting the radiation emitted by thetarget as well as the control path may be adapted accordingly. Anexample of irradiating SIL 60 and target 30′ obliquely by a control beam1405 is schematically illustrated in FIG. 16. Target 30′ is irradiatedobliquely to its normal by radiation 1405 via optical component 1410.Radiation 1425 redirected by target 30′ is directed to a detectionarrangement as described herein.

In an embodiment, a plurality of measurement beams for gap control maybe used. For example, there may be a plurality of beams provided, forexample, according to the arrangement of FIGS. 14 and 15. There may be aplurality of beams provided, for example, according to the arrangementof FIG. 16. Or, there may be provided a combination of one or more beamsprovided, for example, according to the arrangement of FIGS. 14 and 15,and one or more beams provided, for example, according to thearrangement of FIG. 16.

FIG. 17 is a flow chart showing an example method of determining andcontrolling a gap between components in an optical apparatus. The methodin general is implemented by optical and electronic hardware components,in combination with suitable programming instructions provided to aprocessing system. The gap may be, e.g., a gap between a high numericalaperture optical arrangement and a reflective or diffractive surface,referred to in this example as the target surface. The high numericalaperture optical arrangement may be, for instance, the opticalarrangement comprising objective 15 and SIL 60 and the reflective ordiffractive surface may be for instance target 30 or 30′. The gap maymore generally be between any two components in an optical system.

The method comprises the following steps:

S101: A target structure comprising for example a metrology target on asubstrate is positioned, at a predefined position in the X-Y-Zdirections, relative to the optical arrangement. A ‘coarse’ positioning(with an accuracy of the order of mm or microns) of the diffractivesurface relative to the high numerical aperture optical arrangement maybe performed using other sensors, if necessary to set the gap value foruse with, e.g., one or more radiation beams for finer control.Conventional substrate supports and positioning systems can be used forthis step. A ‘fine’ positioning controls the gap by following the stepsdescribed, below.

S102: One or more radiation measurement beams are directed through theoptical arrangement onto the target surface.

S103: The radiation redirected by the target is collected by the opticalarrangement and directed to one or more detector arrangements, such asdescribed above. The detector arrangement(s) produces one or moredetection signals based on the radiation received. In an embodiment,there is obtained a first signal by a first position measurement process(e.g., a signal measured as described in the embodiments of FIGS. 9-13)and there is obtained a second signal by a second position measurementprocess different than the first position measurement process (e.g., aGES signal). In an embodiment, the measurement of S103 may compriseproviding radiation through the optical component to reach the surface;blocking at least part of the radiation redirected by the surface tocause a change of shape or size of illuminated area in a pupil, orconjugate thereof, as a function of change in position between theoptical component and the surface; and detecting the redirectedradiation of the illuminated area.

S104: A processing system analyses the one or more detection signals anddetermines, e.g., a distance value of gap 795, a brake, trigger or othersignal as described herein and/or one or more setpoints for movement ofa component. The processing system may further store the analysisoutput. Steps S102-S104 would be repeated as long as gap control usingthe measurement beam(s) is needed. For example, in an embodiment, thedetected redirected radiation of the illuminated area may be used toproduce a trigger signal based on which the position of the opticalcomponent with respect to the surface is controlled.

S105: The gap distance 795 is controlled using, or based on, an outputof step S104. For example, a distance value of gap 795 may be comparedto a set value, and processor PU may then issue commands to cause changein the relative position between one or more parts of the opticalarrangement and the target surface. In the example of the inspectionapparatus of FIG. 6, the distance of gap 795 may be adjusted usingactuator 66. In an embodiment, there is provided control of relativemovement between the optical component (e.g., SIL 60) and the surface(e.g., surface W) for a first range of motion (e.g., to setpoint 810and/or 820) using the first signal described above and there is providedcontrol of relative movement between the optical component and thesurface for a second range of motion (e.g., to setpoint 830) using thesecond signal as described above, the second range of motion beingnearer the surface than the first range of motion.

Thus, in an embodiment, there is provided an at least two-step brakingsystem or process for near-field metrology/inspection. There is provideda detection/trigger signal that operates through the objective 15 andSIL 60 (which are already used for metrology/inspection), and that canbe obtained by an already available illumination source, and so mayminimize the added complexity to the optical system. By operatingthrough the SIL 60, the measurement of the distance between surface Wand SIL 60 is performed directly between the SIL 60 and the surface W.

Further, in an embodiment, there is provided a method and system that isrobust to process variations (e.g. different reflection coefficientsfrom different surfaces) because, in an embodiment, the signals from atleast two detectors are compared with respect to each other, and so noabsolute signal may be required. In an embodiment, there is provided afreedom to design the gap distance at which a trigger is caused by, forexample, appropriate design of the size and/or shape of the aperture ofa mask located in front of a detector of the system and/or appropriatedesign of the (de)magnification of the conjugated pupil plane.

While embodiments herein have been discussed mostly in relation toapproaching surface W (e.g., a surface of target 30/30′), the techniquesand apparatus discussed herein may also be used for retraction and/ormaintaining the optical component relative to surface W above a certainminimum height.

For example, productivity may be significantly improved by having aretraction distance of gap 795 for movement between targets, e.g.,second setpoint distance 810 of the gap 795, be reduced as much asfeasible. For example, it may be desirable to reduce the second setpointdistance 810 from the range of about several hundreds of microns to,e.g., in the range of 175-50 microns. The retraction distance of gap 795is selected to help ensure that the SIL 60 does not hit the surface Wduring substantially horizontal motion between the surface W and the SIL60. The retraction distance will depend on numerous variables, such asvariation in temperature, variation in substrate table thickness,variation in the substrate thickness and of one or more process layerson the substrate, variation in the positioning of the substrate stagewith respect to the SIL 60 in z/Rx/Ry (if a tilted substrate moves, theSIL-substrate distance changes during the motion), and/or dynamicvibrations of the substrate stage and SIL 60.

However, many of these variables may change little during substantiallyhorizontal motion between the surface W and the SIL 60 (or when the SIL60 and surface W are essentially still with respect to each other). So,where there is little variation, a retraction distance of severalhundred microns may be too conservative. But, reducing the retractiondistance to in the range of 175-50 microns, e.g., to 50-100 μm or about70 μm, may bring an unacceptable risk of collision. This risk can bemitigated with an additional “far field gap” sensor.

Thus, in an embodiment, a signal similar to as described above is used(and possibly designed for) the retraction height. So, for example,after an inspection or metrology measurement of the surface W, the sizeof gap 795 is increased by the relative movement between the SIL 60 andthe surface W to the desired retraction height (e.g., 175-50 microns,e.g., to 50-100 μm or about 70 μm) using an encoder, interferometer orother sensor (a sensor with a reference at the sensor stage). Then,during relative horizontal motion between the surface W and the SIL 60,a radiation beam is detected through the optical system as describedabove for the multiple-step “braking” process and a trigger signal iscorrespondingly created. So, when the gap reduces in size due to avariation as described above, and reduces to a trigger level e.g., at 50μm where the retraction height is 70 μm, such that a threshold value ispassed for the trigger signal, a trigger can be generated and used toretract, e.g., the SIL 60.

The trigger level should be such that there is sufficient time torespond during relative motion between the surface W and the SIL 60. Itshould also consider the geometry of the bottom of the SIL 60 and thesubstrate topology, e.g., the measured distance through the SIL 60 maynot be the smallest distance between the SIL 60 and the surface W.

In an embodiment, a multi-step safety mechanism may be provided where 1)at a first trigger level (e.g., 50 μm) the distance of the gap betweenthe surface W and the SIL 60 is increased (e.g., the SIL 60 isretracted), and 2) at a second trigger level (e.g., 30 μm), the relativehorizontal motion between the surface W and the SIL 60 is stopped. Thus,the first trigger may allow the relative horizontal motion, whereas thesecond trigger would effectively be an “emergency” stop (e.g., it maycause a delay in processing).

So, in an embodiment, there may be multiple trigger thresholdsassociated with different sizes of gap 795. For example, there may be:

1) a “brake” trigger, with its associated size of the gap 795, asdescribed above for an approach toward the surface W, and/or

2) a retraction trigger, with its associated size of the gap 795, tocause relative substantially vertical motion between the SIL 60 and thesurface W when the gap 795 is at a height for relative horizontal motionbetween the SIL 60 and the surface W (e.g., cause an actuator to moveSIL 60 away from the surface W), and/or

3) an “emergency” trigger, with its associated size of the gap 795, tocause relative horizontal motion between the surface W and the SIL 60 tobe stopped.

In an embodiment, the triggers for 1) and 2), or 1) and 3) may bedetermined from a same signal. To enable multiple trigger levels (i.e.,sizes of gap 795), additional detectors with different maskconfigurations may be provided. In an embodiment using a detector and a“software” mask, a different software mask may be employed at differentpoints of the relative position between the SIL 60 and the surface W.For example, a first software mask may be used for 1), a second softwaremask used for 2) and a third software mask used for 3).

In an embodiment, to provide the control described herein, there may belittle, or no, impact on mechanical hardware, there may be limitedimpact on optical hardware, and there may be limited impact on motioncontrol software through extension of a set point generator andapplicable signal processing.

As described above, in an embodiment, there were provided varioustechniques to control the gap by a technique based on one or morespecific signals. The techniques have particular applicability in anoptical metrology or inspection apparatus such as a scatterometer, analignment sensor (which determine alignment between alignment mark), anencoder or interferometer (which enable position measurement), and/or aheight or level sensor (which enables measuring of the position of asurface), but can be applied in other applications of SILs or in otherapplications where an object is positioned and/or maintained very closeto another object (e.g., in the below 400 nm range). The technique neednot be applied exclusively, and could be applied in combination with oneor more other techniques, including one or more techniques discussed inthe cited documents.

While the various embodiments herein primarily describe position controlof a SIL relative to a substrate/target surface, the disclosed methodsand apparatus may be used to control the position of any component, suchas a microcantilever, relative to any surface.

Reference to the gap is not intended to imply that a medium between SIL60 and target 30 must be, e.g., air, or even that it must be gaseous.The medium within the gap in any particular implementation may be avacuum or partial vacuum, any gaseous or liquid medium, whose refractiveindex meets the requirements of the optical functions of the apparatus.

In an embodiment, there is provided a method of position control of anoptical component relative to a surface, the method comprising:obtaining a first signal by a first position measurement process;controlling relative movement between the optical component and thesurface for a first range of motion using the first signal; obtaining asecond signal by a second position measurement process different thanthe first position measurement process; and controlling relativemovement between the optical component and the surface for a secondrange of motion using the second signal, the second range of motionbeing nearer the surface than the first range of motion.

In an embodiment, obtaining the first signal comprises: providingradiation through the optical component to reach the surface; blockingat least part of the radiation redirected by the surface to cause achange of shape or size of illuminated area in a pupil, or conjugatethereof, as a function of change in position between the opticalcomponent and the surface; and detecting the redirected radiation of theilluminated area to produce a signal used to derive the first signal. Inan embodiment, the detecting comprises: detecting, using a firstdetector, at least part of the redirected radiation after having passedthrough an aperture of a first mask to produce a first detection signal;detecting, using a second detector, at least part of the redirectedradiation after having passed through an aperture of a second mask toproduce a second detection signal; and deriving the first signal as afunction of the first and second detection signals. In an embodiment,the first mask comprises an aperture located at the intersection of anoptical axis of the redirected radiation with the first mask and thesecond mask comprise an aperture spaced apart from the intersection ofthe optical axis with the second mask and having an inner peripheryfurther from the optical axis than an outer periphery of the aperture ofthe first mask. In an embodiment, the detecting comprises: detecting,using a detector, at least part of the redirected radiation after havingpassed through an aperture of a mask to produce a detection signal, theaperture spaced apart from the intersection of an optical axis of theredirected radiation with the mask; and deriving the first signal as afunction of a filtered version of the detection signal. In anembodiment, the detecting comprises: detecting, using a first detector,at least part of the redirected radiation to produce a first detectionsignal; detecting, using a second detector, at least part of theredirected radiation to produce a second detection signal, wherein thefirst detector has a first detector radiation receiving elementextending in a plane and the second detector has a second detectorradiation receiving element extending in substantially the same plane asthe first detector radiation receiving element and the first detectorradiation receiving element being generally concentric to the seconddetector radiation receiving element; and deriving the first signal as afunction of the first and second detection signals. In an embodiment,the first signal is a function of the second detection signal divided bythe first detection signal. In an embodiment, the method furthercomprises evaluating the first signal against a threshold and upon thefirst signal passing the threshold, stopping, or beginning to stop, therelative movement between the optical component and the surface in thefirst range of motion. In an embodiment, the second signal is a gaperror signal (GES). In an embodiment, the method comprises: providingradiation through the optical component to reach the surface; detectingradiation redirected by the surface to produce a signal representativeof a size of a gap between the optical component and the surface; andevaluating the signal against a threshold and upon the signal passingthe threshold, causing a relative movement between the optical componentand the surface to cause in an increase in size of the gap and/orcausing a relative horizontal motion between the optical component andthe surface to stop.

In an embodiment, there is provided a method of position control of anoptical component relative to a surface, the method comprising:providing radiation through the optical component to reach the surface;blocking at least part of the radiation redirected by the surface tocause a change of shape or size of illuminated area in a pupil, orconjugate thereof, as a function of change in position between theoptical component and the surface; and detecting the redirectedradiation of the illuminated area to produce a trigger signal based onwhich the position of the optical component with respect to the surfaceis controlled.

In an embodiment, the detecting comprises: detecting, using a firstdetector, at least part of the redirected radiation after having passedthrough an aperture of a first mask to produce a first detection signal;detecting, using a second detector, at least part of the redirectedradiation after having passed through an aperture of a second mask toproduce a second detection signal; and deriving the trigger signal as afunction of the first and second detection signals. In an embodiment,the first mask comprises an aperture located at the intersection of anoptical axis of the redirected radiation with the first mask and thesecond mask comprise an aperture spaced apart from the intersection ofthe optical axis with the second mask and having an inner peripheryfurther from the optical axis than an outer periphery of the aperture ofthe first mask. In an embodiment, the detecting comprises: detecting,using a detector, at least part of the redirected radiation after havingpassed through an aperture of a mask to produce a detection signal, theaperture spaced apart from the intersection of an optical axis of theredirected radiation with the mask; and deriving the trigger signal as afunction of a filtered version of the detection signal. In anembodiment, the detecting comprises: detecting, using a first detector,at least part of the redirected radiation to produce a first detectionsignal; detecting, using a second detector, at least part of theredirected radiation to produce a second detection signal, wherein thefirst detector has a first detector radiation receiving elementextending in a plane and the second detector has a second detectorradiation receiving element extending in substantially the same plane asthe first detector radiation receiving element and the first detectorradiation receiving element being generally concentric to the seconddetector radiation receiving element; and deriving the trigger signal asa function of the first and second detection signals. In an embodiment,the trigger signal is a function of the second detection signal dividedby the first detection signal. In an embodiment, the method furthercomprises evaluating the trigger signal against a threshold and upon thetrigger signal passing the threshold, stopping, or beginning to stop,the movement of the optical component in the first range of motion.

In an embodiment, there is provided a method comprising: providingradiation through an optical component to reach a surface; causing achange of shape or size of an area illuminated by radiation redirectedby the surface as a function of change in position between the opticalcomponent and the surface; detecting, using a detector, at least part ofthe redirected radiation after having passed through an aperture of amask to produce a detection signal, the aperture spaced apart from theintersection of an optical axis of the redirected radiation with themask; and deriving a trigger signal as a function of a filtered versionof the detection signal.

In an embodiment, the trigger signal is a function of the detectionsignal divided by the filtered version of the detection signal. In anembodiment, the method further comprises evaluating the trigger signalagainst a threshold and upon the trigger signal passing the threshold,stopping, or beginning to stop, a relative movement between the opticalcomponent and the surface. In an embodiment, the filtered version of thedetection signal comprises a low-pass version of the detection signal.

In an embodiment, there is provided a method comprising: providingradiation through an optical component to reach a surface; causing achange of shape or size of an area illuminated by radiation redirectedby the surface as a function of change in position between the opticalcomponent and the surface; detecting, using a first detector, at leastpart of the redirected radiation to produce a first detection signal;detecting, using a second detector, at least part of the redirectedradiation to produce a second detection signal, wherein the firstdetector has a first detector radiation receiving element extending in aplane and the second detector has a second detector radiation receivingelement extending in substantially the same plane as the first detectorradiation receiving element and the first detector radiation receivingelement being generally concentric to the second detector radiationreceiving element; and deriving a trigger signal as a function of thefirst and second detection signals. In an embodiment, the method furthercomprises evaluating the trigger signal against a threshold and upon thetrigger signal passing the threshold, stopping, or beginning to stop, arelative movement between the optical component and the surface.

In an embodiment, the optical component comprises a solid immersion lensand the surface comprises a measurement target surface. In anembodiment, the method further comprises positioning the opticalcomponent within 1 nm to 400 nm of the surface.

In an embodiment, there is provided a method of manufacturing deviceswherein a device pattern is applied to a series of substrates using alithographic process, the method including inspecting at least a targetformed as part of or beside the device pattern on at least one of thesubstrates using a method as described herein, and controlling thelithographic process for later substrates in accordance with the resultof the method.

In an embodiment, there is provided a non-transitory computer programproduct comprising machine-readable instructions for causing a processorto cause performance of a method as described herein.

In an embodiment, there is provided a system comprising: an inspectionapparatus configured to provide a beam on a measurement target on asubstrate and to detect radiation redirected by the target to determinea parameter of a lithographic process; and a non-transitory computerprogram product as described herein. In an embodiment, the systemfurther comprises a lithographic apparatus comprising a supportstructure configured to hold a patterning device to modulate a radiationbeam and a projection optical system arranged to project the modulatedonto a radiation-sensitive substrate.

In an embodiment, there is provided a detection apparatus comprising: afirst mask configured to receive at least part of radiation redirectedfrom a surface and passing through an optical component moving relativeto the surface, the first mask having an aperture to allow radiation topass therethrough; a first detector configured to receive redirectedradiation passing through the first mask to produce a first detectionsignal; a second mask configured to receive at least part of theredirected radiation, the second mask having an aperture to allowradiation to pass therethrough, wherein the first mask comprises anaperture located at the intersection of an optical axis of theredirected radiation with the first mask and the second mask comprisesan aperture spaced apart from the intersection of the optical axis withthe second mask and having an inner periphery further from the opticalaxis than an outer periphery of the aperture of the first mask; and asecond detector configured to receive redirected radiation passingthrough the second mask to produce a second detection signal.

In an embodiment, the apparatus further comprises a control systemconfigured to produce a trigger signal based on which the position ofthe optical component with respect to the surface is controlled, thetrigger signal being a function of the first and second detectionsignals. In an embodiment, the apparatus further comprises a beamsplitter configured to receive redirected and to provide at least partof the redirected radiation to the first mask and at least part of theredirected radiation to the second mask. In an embodiment, the opticalcomponent comprises a solid immersion lens and the surface comprises ameasurement target surface.

In an embodiment, there is provided a detection apparatus comprising: afirst detector configured to detect radiation, the first detector havinga first detector radiation receiving element extending in a plane; and asecond detector configured to detect radiation, the second detectorhaving a second detector radiation receiving element extending insubstantially the same plane as the first detector radiation receivingelement and the first detector radiation receiving element beinggenerally concentric to the second detector radiation receiving element.

In an embodiment, the first and second radiation receiving elements areinsulated from each other. In an embodiment, the second radiationreceiving element substantially surrounds the first radiation receivingelement.

In an embodiment, there is provided a detection apparatus comprising: amask configured to receive at least part of radiation redirected from asurface and passing through an optical component moving relative to thesurface, the mask having an aperture spaced apart from the intersectionof an optical axis of the redirected radiation with the mask; a detectorconfigured detect at least part of the redirected radiation after havingpassed through the aperture of the mask to produce a detection signal;and a control system configured to produce a trigger signal based onwhich the position of the optical component with respect to the surfaceis controlled, the trigger signal being a function of a filtered versionof the detection signal.

In an embodiment, the optical component comprises a solid immersion lensand the surface comprises a measurement target surface. In anembodiment, the filtered version of the detection signal comprises alow-pass version of the detection signal.

In an embodiment, there is provided a detection apparatus comprising: adetector configured detect at least part of radiation redirected from asurface and passing through an optical component moving relative to thesurface to produce a detection signal, wherein a shape or size ofilluminated area in a pupil, or conjugate thereof, changes as a functionof change in position between the optical component and the surface; anda processor system configured to apply a software mask having anaperture spaced apart from the intersection of an optical axis of theredirected radiation with the detector to effectively block processingof radiation received by the detector nearer to the optical axis thanthe aperture, to produce a trigger signal based on which the position ofthe optical component with respect to the surface is controlled.

The algorithms described in this document may be implemented via codingof a suitable software program to be performed by, e.g., processor PU orits equivalent in the form of a dedicated microprocessor or the like.

Any controllers or control systems described herein may each or incombination be operable when the one or more computer programs are readby one or more computer processors located within at least one componentof the lithographic apparatus. The controllers or control systems mayeach or in combination have any suitable configuration for receiving,processing, and sending signals. One or more processors are configuredto communicate with the at least one of the controllers or controlsystems. For example, each controller or control system may include oneor more processors for executing the computer programs that includemachine-readable instructions for the methods described above. Thecontrollers or control systems may include a data storage medium forstoring such computer programs, and/or hardware to receive such medium.So the controller(s) or control system(s) may operate according themachine readable instructions of one or more computer programs.

Although specific reference may have been made in this text to the useof embodiments of the invention in the context of metrology orinspection apparatus used to inspect or measure items in associationwith, e.g., optical lithography, it will be appreciated that the methodsand apparatus described herein may be used in other applications, forexample imprint lithography, the use or manufacture of integratedoptical systems, the use or manufacture of guidance and detectionpatterns for magnetic domain memories, the use or manufacture offlat-panel displays, the use or manufacture of liquid-crystal displays(LCDs), the use or manufacture of thin film magnetic heads, etc. Thesubstrate referred to herein may be processed, before or after exposure,in for example a track (a tool that typically applies a layer of resistto a substrate and develops the exposed resist), a metrology tool and/oran inspection tool. Where applicable, the disclosure herein may beapplied to such and other substrate processing tools. Further, thesubstrate may be processed more than once, for example in order tocreate a multi-layer IC, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of less than about 400 nm and greater than about 20nm, or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet(EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), aswell as particle beams, such as ion beams or electron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, an embodiment of the invention may takethe form of a computer program containing one or more sequences ofmachine-readable instructions describing a method as disclosed herein,or a non-transitory data storage medium (e.g. semiconductor memory,magnetic or optical disk, etc.) or a transitory medium having such acomputer program therein. Further, the machine readable instruction maybe embodied in two or more computer programs. The two or more computerprograms may be stored on one or more different data storage media.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

What is claimed is:
 1. A method of position control of an opticalcomponent relative to a surface, the method comprising: obtaining afirst signal by a first position measurement process; controllingrelative movement between the optical component and the surface for afirst range of motion using the first signal; obtaining a second signalby a second position measurement process different than the firstposition measurement process; and controlling relative movement betweenthe optical component and the surface for a second range of motion usingthe second signal, the second range of motion being nearer the surfacethan the first range of motion.
 2. The method of claim 1, whereinobtaining the first signal comprises: providing radiation through theoptical component to reach the surface; blocking at least part of theradiation redirected by the surface to cause a change of shape or sizeof illuminated area in a pupil, or conjugate thereof, as a function ofchange in position between the optical component and the surface; anddetecting the redirected radiation of the illuminated area to produce asignal used to derive the first signal.
 3. The method of claim 2,wherein the detecting comprises: detecting, using a first detector, atleast part of the redirected radiation after having passed through anaperture of a first mask to produce a first detection signal; detecting,using a second detector, at least part of the redirected radiation afterhaving passed through an aperture of a second mask to produce a seconddetection signal; and deriving the first signal as a function of thefirst and second detection signals.
 4. The method of claim 3, whereinthe first mask comprises an aperture located at the intersection of anoptical axis of the redirected radiation with the first mask and thesecond mask comprise an aperture spaced apart from the intersection ofthe optical axis with the second mask and having an inner peripheryfurther from the optical axis than an outer periphery of the aperture ofthe first mask.
 5. The method of claim 2, wherein the detectingcomprises: detecting, using a detector, at least part of the redirectedradiation after having passed through an aperture of a mask to produce adetection signal, the aperture spaced apart from the intersection of anoptical axis of the redirected radiation with the mask; and deriving thefirst signal as a function of a filtered version of the detectionsignal.
 6. The method of claim 2, wherein the detecting comprises:detecting, using a first detector, at least part of the redirectedradiation to produce a first detection signal; detecting, using a seconddetector, at least part of the redirected radiation to produce a seconddetection signal, wherein the first detector has a first detectorradiation receiving element extending in a plane and the second detectorhas a second detector radiation receiving element extending insubstantially the same plane as the first detector radiation receivingelement and the first detector radiation receiving element beinggenerally concentric to the second detector radiation receiving element;and deriving the first signal as a function of the first and seconddetection signals.
 7. The method of claim 3, wherein the first signal isa function of the second detection signal divided by the first detectionsignal.
 8. The method of claim 1, further comprising evaluating thefirst signal against a threshold and upon the first signal passing thethreshold, stopping, or beginning to stop, the relative movement betweenthe optical component and the surface in the first range of motion. 9.The method of claim 1, wherein the second signal is a gap error signal(GES).
 10. The method of claim 1, comprising: providing radiationthrough the optical component to reach the surface; detecting radiationredirected by the surface to produce a signal representative of a sizeof a gap between the optical component and the surface; and evaluatingthe signal against a threshold and upon the signal passing thethreshold, causing a relative movement between the optical component andthe surface to cause in an increase in size of the gap and/or causing arelative horizontal motion between the optical component and the surfaceto stop.
 11. The method of claim 1, wherein the optical componentcomprises a solid immersion lens and the surface comprises a measurementtarget surface.
 12. The method of claim 1, further comprisingpositioning the optical component within 1 nm to 400 nm of the surface.13. A method of manufacturing devices wherein a device pattern isapplied to a series of substrates using a lithographic process, themethod including inspecting at least a target formed as part of orbeside the device pattern on at least one of the substrates using themethod of claim 1, and controlling the lithographic process for latersubstrates in accordance with the result of the method.
 14. Anon-transitory computer program product comprising machine-readableinstructions for causing a processor to cause performance of the methodof claim
 1. 15. A system comprising: an inspection apparatus configuredto provide a beam on a measurement target on a substrate and to detectradiation redirected by the target to determine a parameter of alithographic process; and the non-transitory computer program product ofclaim
 14. 16. The system of claim 15, further comprising a lithographicapparatus comprising a support structure configured to hold a patterningdevice to modulate a radiation beam and a projection optical systemarranged to project the modulated onto a radiation-sensitive substrate.17. A method of position control of an optical component relative to asurface, the method comprising: providing radiation through the opticalcomponent to reach the surface; blocking at least part of the radiationredirected by the surface to cause a change of shape or size ofilluminated area in a pupil, or conjugate thereof, as a function ofchange in position between the optical component and the surface; anddetecting the redirected radiation of the illuminated area to produce atrigger signal based on which the position of the optical component withrespect to the surface is controlled.
 18. The method of claim 17,wherein the detecting comprises: detecting, using a first detector, atleast part of the redirected radiation after having passed through anaperture of a first mask to produce a first detection signal; detecting,using a second detector, at least part of the redirected radiation afterhaving passed through an aperture of a second mask to produce a seconddetection signal; and deriving the trigger signal as a function of thefirst and second detection signals.
 19. A method comprising: providingradiation through an optical component to reach a surface; causing achange of shape or size of an area illuminated by radiation redirectedby the surface as a function of change in position between the opticalcomponent and the surface; detecting, using a detector, at least part ofthe redirected radiation after having passed through an aperture of amask to produce a detection signal, the aperture spaced apart from theintersection of an optical axis of the redirected radiation with themask; and deriving a trigger signal as a function of a filtered versionof the detection signal.
 20. The method of claim 19, wherein the triggersignal is a function of the detection signal divided by the filteredversion of the detection signal.